If you know the segments are available and you can get them to appear individually on lifemiles.com, then you should take screenshots and then email them to Lifemiles, including your name, DOB, the flights and dates, and they will manually hold the award for you. They'll then call you for payment details.

If the segments don't show up at all, well, I've never tried to proceed in that case, but I would email them if the segments are showing up on UA or AC but not AV's site.